Research On Novel Sensor Based On Strong Coupling Multi-Core Fiber Bragg Grating

Multi-core fiber（MCF）has attracted a lot of attention from scholars in the field of fiber optic sensing in recent years because of its many advantages such as small size,compact structure,spatial diversity and self-temperature compensation.The vast majority of traditional MCF-based sensors or sensing systems use non-/weakly coupled MCFs,thus inevitably requiring the use of special fan-in/fan-out devices to extract sensing information from each core channel in the MCF,which undoubtedly increases the complexity and cost of the sensing and demodulation system.In order to solve the problems of existing MCF sensor devices or systems,this thesis carries out research on sensors based on strongly coupled multicore fiber（SCMCF）,combining fiber Bragg grating（FBG）with SCMCF for the first time,and designing and verifying two novel fiber sensors,the specific research results are as follows.In response to the curvature-temperature cross-sensitivity problem of conventional curvature sensors,a novel temperature-insensitive fiber optic curvature sensor is proposed in this thesis.The sensor inscribes a 2 mm long low-reflectivity FBG into the SCMCF and achieves extraction of sensing information by fusing one end of it to a standard single-mode fiber（SMF）.Since the center core of the SCMCF used supports only two non-zero intensity supermode transmissions,the reflection spectrum of the FBG contains two resonance peaks and a deep notch is formed between the two resonance peaks due to interference effects.The experimental results show that the depth of this notch varies monotonically with the curvature,and the sensitivity of the designed curvature sensor can reach 15.9 d B/m^1-in the low curvature measurement range.In addition,the notch depth of this sensor hardly varies with temperature,so the demodulation method that measures the notch depth enables temperature-insensitive curvature demodulation with a cross-sensitivity of less than0.001 m^-1/?C between temperature and curvature.To realize multipoint and multiparameter sensing based on SCMCF,FBG is first inscribed in a section of SCMCF,and then its two ends are fused with SMF to construct an interferometer structure based on supermode interference.The experimental results show that the FBG inscribed in SCMCF has a good linear response to temperature as well as transverse load,with sensitivities of 9.77 pm/?C and-157.3 pm/（N/mm）,respectively.By cascading FBGs of different Bragg wavelengths using WDM technology,multi-point sensing along the length of the fiber can be achieved;by measuring the periodic displacement of the interference spectrum through the interferometer structure,which is sensitive to vibration,high-precision vibration monitoring can be achieved.Since the periodic variation of the interference spectrum is measured in vibration sensing instead of the absolute variation of wavelength,the problem of cross-sensitivity can be effectively eliminated,which is conducive to the realization of multiparameter sensing.